This invention relates to power management and power conservation techniques for personal computers. More particularly, this invention relates to power conservation in the monitor of a personal computer system.
In recent years, power management features in personal computers have come into more and more common use, particularly in the very small, battery operated notebook computers. An example of an apparatus for reducing power consumption in a computer system is shown in U.S. Pat. No. 4,980,836, which is assigned to the assignee of the present invention and incorporated herein by reference. This apparatus shuts down part of the computer system after a period of inactivity, thus conserving electrical power.
Other ways of reducing power consumption may involve turning off, or slowing down, only certain internal peripheral devices, such as the hard drive. Usually, these methods monitor activity to the peripheral device and, after a certain period of inactivity, shut the peripheral down. Typical power conservation for powering down a hard drive may result in approximately a three percent reduction in power usage by the entire computer system.
In most personal computer systems, the monitor is one of the biggest users of power. However, no one has yet found a way to provide power management features in a computer monitor such as the cathode ray tube (CRT) monitors commonly used with most desk top personal computers. These monitors are turned on all the time the computer is in use and draw full power the entire time. In a typical system, the monitor may consume three-fourths of all the power. Therefore, considerable power savings may be had if some way to power down the monitor could be accomplished.
In a typical CRT monitor, a filament produces electrons when it becomes very hot. A series of grids and anodes strips the electrons from the filament, accelerates the electrons into a beam and focuses the beam onto a phosphorescent screen. A set of coils deflects the beam onto different areas of the screen to produce an image. The filament, the grids and a first anode form an electron gun. A first grid modulates the number of electrons in the electron beam. A second grid accelerates the electrons through the electron gun. A third grid focuses the electron beam very tightly. The first anode accelerates the electrons even more before they leave the electron gun. Then the coils deflect the beam in the horizontal and vertical directions. Finally, a second anode, a very high voltage device, accelerates the beam a final time before the electrons strike the phosphors on the screen causing an emission of photons. Control circuitry provides the control signals to synchronize the operation of each of these elements so that an image appears on the screen. The deflection coils and the second anode are by far the biggest drain on the power supply.
When a user turns on a monitor, there is usually a several second delay before an image appears on the screen. This delay is due to the need to heat up the heating filament of the electron gun. Although, the deflection coils and the second anode draw the most power, they can be turned on much more quickly than can the heating element. The monitor's control circuitry and the electron gun, except for the filament, can also be turned on very quickly. Therefore, the limiting factor in turning on a monitor is the time needed to heat up the heating filament.
Additionally, the limiting factor in the useful life of a CRT monitor is usually the heating filament. In other words, keeping the filament hot will eventually cause the filament to wear out, usually before the other elements of the monitor wear out. The ratio of the life of the filament to its temperature is governed by the root mean square distribution. Thus, an increase in the life of a filament will not be linearly proportional to a reduction in the temperature of the filament. Instead, the increase in the life of the filament will be proportional to the square of the reduction in its temperature, so a relatively small change in the temperature of the filament will result in a relatively large change in its useful life. Also, the ratio of the change in voltage across the filament to the change in temperature of the filament is governed by the root mean square distribution. In this case, the change in voltage is proportional to the square of the change in temperature. Therefore, a reduction in the voltage across the filament will result in a relatively small reduction in the temperature of the filament, which will result in a relatively large increase in its useful life.
Televisions made several years ago powered down every part of the television system when the television was turned off. Then when the television was turned on, several seconds would pass before an image would appear on the screen, because the system had to wait for the heating element to heat up. However, most televisions made today only require a short wait before an image appears. This short time period is due to the fact that most televisions no longer power down every element of the system when the television is turned off. Instead, the new televisions keep the heating element hot even when the system is off. Thus, if a person were to touch the back of a modern television, that person would find the television to be quite warm even though it had been left off for several hours.
Many screen-saver programs already exist for personal computers. These programs usually remove the image on the screen of the monitor after a period of user inactivity and replace the image with a moving image that will not "burn" into screen. One type of screen-saver blanks the screen entirely, so that there is no image at all. When the user touches a key on the keyboard or moves a mouse pointer, thus resuming activity, the previous image is restored to the screen. However, screen-saver programs do not actually conserve any power in the system. Instead, all of the elements of the monitor remain under full power, even if the screen has been completely blanked out.
Therefore, it is an object of the present invention to provide a power management feature for a personal computer system by powering down selected parts of the system's monitor.